1. Field of the Invention
The present invention relates to a thermocouple lance with a protective sheath suitable for measuring relatively high temperatures in molten metal baths of steel or the like.
2. Description of the Prior Art
Conventionally used for measuring a high temperature of about 1700.degree. C. encountered in a molten steel bath are thermocouples made from wires of Pt--Rh alloy relatively high in melting point and stable in the atmosphere, the wires being fitted in a tubular sheath of aluminosilicate fibers. The prior thermocouples are currently expensive as well as less in the life expectancy of repetitive use because they are apt to become not capable of accurate temperature measurement and thus have to be disposed after only one or two measurements in the molten steel bath.
A type of the prior sheathed thermocouples has W--Re alloy wires that are enveloped in the metallic protective tubular sheath made of, for example, stainless steel so as to be adapted for use in high temperatures. Any type of the thermocouples with stainless steel protective sheaths has been designed to be used in the atmosphere of above 1000.degree. C. This protective sheath is usually produced from a special heat resisting alloy of, for example, Inconel which is a nickel-base alloy including chrome and iron. Another type of the prior thermocouples is well known in which the Pt--Rh wires are encapsulated in the protective tube of cermet.
Disclosed in Japanese Patent Laid-Open No. 160200/1994 is a sheathed thermocouple assembly with a hermetically sealing plug, which may be free from errors in its measured temperature resulting from a temperature gradient occurring in the sealing plug owing to transient variations in temperature. The sheathed thermocouple assembly includes therein an alumel-chromel thermocouple consisting of two wires of different metals, or alumel and chromel. The two wires are contained in the stainless steel sheath together with inorganic insulating material electrically insulated in relation from each other and further hermetically sealed at the open end of the sheath by means of the plug. The plug is provided in a ceramic end plate thereof with two tubes, which are made of Kovar, or iron-nickel-cobalt alloy, and coated on the inner surfaces thereof with electrically insulating sleeves. The two wires extend outside the sheath through tubes with no direct contact with their associated tubes.
On the other hand, the thermal shock resistance of the cermet-made protective sheath is 1.5 times of that of the Si.sub.3 N.sub.4 -made protective sheath. Moreover, the Si.sub.3 N.sub.4 -made protective sheath, when directly immersed into the molten steel bath above 1700.degree. C., may crack in a relatively short interval of immersion resulting in breaking the sheath.
It is to be noted that the Pt--Rh thermocouple may not be used in an inert-gas atmosphere and its permissible temperature limit in an environmental atmosphere is at 1500.degree. C.
In contrast, the W--Re thermocouple may be used in any of inert-gas atmosphere and environmental atmosphere and its permissible temperature limit is at 400.degree. C. in environmental atmosphere while at 2300.degree. C. in inert-gas atmosphere.
The Pt--Rh thermocouple of platinum-rhodium alloy wires has usually the thermo-electromotive force that is 1/15 times of that of the alumel-chromel thermocouple while 1/7 times of that of the W--Re thermocouple. Hence, the Pt--Rh thermocouple has a shortcoming which is inferior in accuracy as well as rapid response of the temperature measurement when compared with the other types of the thermocouple.
In order to measure the temperature of the molten bath at the site of the furnaces by means of the prior thermocouples described above, consequently, the operator has been compelled to stay near the furnaces for about eight seconds until the measured temperature becomes stable.
The prior thermocouples have further disadvantage in which they are apt to be adhered to by molten metals so as to be deteriorated in rapid response of temperature measurement. This causes a collateral problem in which troublesome process is inevitably required to remove iron or steel adhering to the Pt--Rh alloy wires and protective sheath of the thermocouple assembly. Another problem faced in concurrently using thermocouples is the need to frequently replace the thermocouples because they are apt to become not capable of taking accurate temperature measurements after one or two uses in the molten steel bath. It is to be understood that the W--Re alloy wires for the thermocouples tend to be subjected to oxidation in environmental atmosphere so that it is not permitted to use for measuring high temperatures in the molten iron for casting. The outer protective sheath is also disadvantageously apt to be adhered to by molten metals.